1. Field of Invention
The invention relates generally to a flow-control passive valve for controlling flow of a fluid between a high-pressure upstream space and a low-pressure downstream space.
2. Description of Related Art
Damping-adjustable shock absorbers, which are able to change their damping characteristics under control of an electronic-control unit to modify the behavior of the vehicle suspension system depending, for instance, on the road conditions and/or on the vehicle driving conditions, are nowadays more and more often used, particularly in the automotive field.
With reference to FIG. 1 of the attached drawing, a damping-adjustable shock absorber according to a known embodiment is generally indicated 10 and basically comprises:
a pressure tube 12 enclosing a pressure chamber 14 filled with a damping fluid (typically oil);
a plunger 16 that is slidably mounted inside the pressure chamber 14 of the pressure tube 12 and divides it into a lower pressure chamber 14a and an upper pressure chamber 14b; 
a rod 18 that carries at an end thereof the plunger 16 and projects on the opposite side from the pressure tube 12;
an outer tube 20;
an intermediate tube 22 that is fitted onto the pressure tube 12 and encloses a by-pass chamber 24 that communicates with the upper pressure chamber 14b through communication holes 26 provided in the pressure tube 12; and
a flow-control active valve 28 (typically a solenoid valve) that is connected to the intermediate tube 22 and is arranged to control the flow of the damping fluid between the pressure chamber 14 and the by-pass chamber 24.
FIG. 2 of the attached drawing shows a typical “pressure-flow-rate characteristic” curve of the damping-adjustable shock absorber according to the prior art illustrated in FIG. 1, with different values of the driving current of the flow-control solenoid valve. As can be seen, the “pressure-flow-rate characteristic” curves have each a first ascending section (low flow-rate values), the gradient of which increases along with the driving current of the solenoid valve, and a second section (high flow-rate values) that is also ascending, the gradient of which is lower than that of the first section. In an embodiment, “pressure-flow-rate characteristic” curves have a first ascending section and a constant, if not even descending, second section adjacent to the first one.
An example of a “pressure-flow-rate characteristic” curve is shown in FIG. 3 of the attached drawing, where “α” indicates the gradient of the first ascending section—in particular, a substantially linearly ascending section—of the curve (from a flow-rate value equal to zero to a flow-rate value indicated “Q*”) and “p*” indicates the constant value of the pressure starting from the flow-rate value “Q*.” Nowadays, in order to obtain an operation of the shock absorber that is as closest as possible to the ideal one represented by the “pressure-flow-rate characteristic” curve of FIG. 3, the flow-control solenoid valve can be operated with suitable driving logics. It is, thus, possible to obtain a second section of the “pressure-flow-rate characteristic” curve that has a lower gradient than that of the first section, but that is still typically ascending instead of constant.
It is an object of the invention to allow to obtain a “pressure-flow-rate characteristic” curve in which a first ascending section is immediately followed by a second constant section, as in the curve shown in FIG. 3, or even by a second descending section.